The present invention relates generally to an image-pickup apparatus, such as a video camera and a digital still camera, and more particularly to focus control in the image-pickup apparatus.
In the autofocus (“AF”) control in a video camera and the like, a TV-AF method is mainstream, which generates an AF evaluation value signal indicative of sharpness or a contrast state of an image signal generated by using an image-pickup element, and searches for a focus lens position that maximizes the AF evaluation value signal level.
One AF method is an external ranging method or external ranging (or distance-measuring) phase (difference) detection method that provides a ranging sensor independent of an image-pickup lens, operates an in-focus position of a focus lens from a distance to the object detected by the ranging sensor, and moves the focus lens to the in-focus position.
The external phase difference detection method divides a luminous flux from the object into two luminous fluxes, receives a pair of divided rays through a pair of light-receiving element arrays (line sensors), and detects a shift amount between the images formed on the pair of line sensors or a phase difference. Then, this method calculates an object distance based on the phase difference by using the triangulation, and moves a focus lens to an in-focus position corresponding to the object distance.
An internal phase detection method is also known, which divides into two the luminous flux that has passed an exit pupil of an image-pickup lens, and receives the two divided rays through a pair of line sensor. Then, this method detects a phase difference between the two images on the pair of line sensors, calculates a defocus amount of the image-pickup lens from the phase difference, and moves the focus lens by an amount corresponding to the defocus amount.
In order to utilize the high focusing precision of the TV-AF method and the focusing speed of the phase difference detection method, Japanese Patent Laid-Open No. 2005-84426 proposes a hybrid AF method that combines these AF methods. The hybrid AF method proposed in Japanese Patent Laid-Open No. 2005-84426 obtains an in-focus state by using the phase difference detection method, when the TV-AF system does not provide the in-focus state due to low brightness or low contrast of the object.
Japanese Patent Laid-Open No. 05-346536 proposes an image-pickup apparatus that utilizes the phase difference detection method to find a coincidence between image patterns, i.e., the similarity between the two images on the line sensors, and to thereby determine the reliability of the distance measurement result.
However, the conventional hybrid AF method initially provides TV-AF focus control, and then determines whether phase detection focus control is necessary, depending upon a condition of the TV-AF focus control. Therefore, an advantage of the phase difference detection system, i.e., responsive and fast focus control, is unavailable, when a photographer desires.